<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        .div2{
            /*给宽高，要不然就占满屏幕*/
            height: 100px;
            width: 100px;
            background-color: black;
            /*
             position: absolute;相对定位视口(浏览器)
              bottom：在底部
              right：在右边
            */
            position: absolute;
            top: 0;
            left: 0;
        }
    </style>
</head>
<body>

<div class="div2">aa</div>

</body>

</html>